1. Field of the Invention
This invention relates to a ladle furnace for refining molten steel.
2. Description of the Prior Art
As a ladle furnace of this type, for example, a ladle furnace apparatus as shown in FIG. 10 has well been known. In the aforesaid apparatus of FIG. 10, a ladle 10 is provided with an opening 11 at the upper portion thereof and an annular receiving surface 12 for a lid 15' at the periphery thereof. The upper end portion of the ladle 10 may also be formed as shown in FIG. 11. Namely, the ladle 10 is provided with an annular flange 28 at the outer peripheral wall of an upper portion thereof and the upper surface of this flange 28 may construct the lid receiving surface 12. The opening 11 of the ladle 10 is closed by the lid 15'. The lid 15' is suspended from a supporting beam 16 through a chain or wire 16a and an arm 15a fixed to the lid 15'. The supporting beam 16 is subjected to the rising and lowering movement and the horizontal movement by a not illustrated operating means. Therefore, the lid 15' is put on the ladle 10 by operation of the operating means in such a manner an annular lower end surface 17 thereof is mounted on the lid receiving surface 12 of the ladle 10. The lid 15' is provided with electrode holes 18 and through which, electrodes 19 for arc heating extend into the ladle 10. The electrodes 19 are supported by supporting arms 20, respectively. The supporting arms 20 are also subjected to the rising and lowering movement and the horizontal movement by another operating means different from the aforesaid operating means. On the lid 15', there is provided a hood 21 for enveloping the aforesaid electrode holes 18 and a duct 23 for suction connected to the hood 21 is further connected to a blower 25 via a filter 24.
According to the conventional apparatus thus constructed, the lid 15' is taken off and in this state, molten steel tapped from a steelmaking furnace and a slag making material are introduced into the ladle 10 and thereafter, the opening of the ladle 10 is closed by the lid 15'. Then, an electric energy is supplied to the electrodes 19 to generate arc between the electrodes 19 and the molten steel. By this arc, the aforesaid slag making material is slagged to be formed into slag 14 and refining is carried out on the aforesaid molten steel by utilizing the slag 14. In this case, there is injected a reducing atmosphere gas, for example, an inert gas such as argon or the like, into the ladle 10 from an injection port 26 provided at the bottom wall of the ladle 10. The injection port 26, for example, is composed of a porous plug so that it allows injection of the reducing atmosphere gas into the ladle 10. This reducing atmosphere gas brings bubbling action on the molten steel 13 and the slag 14 and stirs them thereby to promote the refining efficiency of slag on the molten steel. In addition, since a space 27 formed above the slag 14 and the molten steel 13 is always filled with the aforesaid reducing gas, there can be prevented lowering of the refining efficiency of slag 14 caused by the oxidation thereof. As another means for injecting the aforesaid reducing atmosphere gas into the ladle 10, the injection port may be provided at the side wall of the ladle 10 or a lance passing through the lid 15' may be inserted into the molten steel 13.
During the aforesaid refining process, a dust containing gas is generated from the molten steel 13 and the slag 14 in the direction of the upper space 27 by the reaction thereof. If the dust containing gas leaks out of the ladle furnace, this results in deterioration of the working environment around the furnace. To prevent this deterioration of the working environment, the interior of the hood 21 is made to be in negative pressure by means of a blower 25. As a result, the aforesaid dust containing gas is sucked into the hood 21 together with the reducing atmosphere gas from the upper space 27 passing through the electrode holes 18. And these gases and dusts pass through the filter 24 so that the dusts are removed therefrom and the resulting gases are emitted to the atmosphere passing through the blower 25.
After completion of the aforesaid refining process, the electrodes 19 and the lid 15' are taken out of the ladle 10. And subsequently, the ladle 10 is inclined to carry out removal of the slag 14 and tap of the molten steel 13. On the other hand, the molten steel may be tapped through a discharge port provided at the bottom wall of the ladle 10, without inclination of the ladle 10.
The aforesaid operations such as to introduce the molten steel and slag into the ladle and to carry out slag refining on the molten steel in the ladle with the atmosphere within the ladle being kept reducing, for example, have been known from the Japanese Industrial Furnace Handbook, on the right column of page 156, lines 9 to 24 (published on July 20, 1978 by Tokyo Technocenter Co., Ltd. in Japan).
A series of processes, comprising the steps of introducing the molten steel and slag into the ladle, refining the molten steel therein and tapping the molten steel from the ladle, is repeatedly performed. Therefore, the slag 14 and the molten steel 13 adhere locally to the lid receiving surface 12 of the ladle 10. In addition, on the lid receiving surface 12, there occurs gradually heat-caused deformation. As a result, if the lid 15' is put on the ladle 10 so as to mount the annular lower end surface 17 of the lid 15' on the lid receiving surface 12 of the ladle 10, it is encountered that a gap 29 of about 30 to 70 mm is locally formed therebetween. If the refining process as described above is performed in such a state, the open air flows into the upper space 27 passing through the aforesaid gap 29, because the upper space 27 is in negative pressure due to the suction of gases by the blower 25. As a result, there are encountered the following disadvantages.
Primarily, the temperature of slag 14 is lowered due to the air flowing into the space 27. Therefore, in order to prevent lowering of the temperature of the aforesaid slag, an electric energy to be supplied to the electrodes 19 has to be increased to increase arc heating. Consequently, electric energy consumption for this purpose is also increased.
Secondarily, the refining efficiency of the slag 14 is remarkably lowered due to oxygen contained in the air entered into the space 27. If the refining efficiency of the slag 14 is lowered, the time required for refining is extended so that damage of the inner wall of the ladle 10 is increased and consequently, a large cost must be expended for repairing the ladle 10. In addition, if the aforesaid refining time is extended, the electric energy to be supplied to the electrodes is much required for preventing lowering of the temperature of slag and molten steel in proportion to the extended time period, resulting in increment of the amount of electric energy required for refining.
Furthermore, in the conventional apparatus described above, the molten steel 13 occasionally reacts with the slag 14 with violence and a large amount of gas is generated at this time. In this case, the gas thus generated is spouted out into the hood 21 through the electrode holes 18 of the lid 15'. And in some cases, it is also encountered that the gas generated leaks out of the apparatus passing through electrode holes 22 of the hood 21. In order to prevent the aforesaid disadvantages, the amount of suction from the hood 21 has to be remarkably increased as much as to exceed the large amount of gas temporally generated as described above. However, if the amount of suck is increased in this manner, the amount of air flowing into the upper space 27 is also increased. In addition, a large capacity of blower must be employed as the blower 25 for sucking the gas so that the operation cost thereof is also increased.
As an art aiming at solving the above mentioned problems, there is an art disclosed, for example, in U.S. Pat. No. 4,460,164. According to this art, a screen 10 which enclosed the upper portion and the side peripheral surface of the lid 15' and is open at the lower end thereof is disposed against the lid 15' with a space therebetween. The screen is provided with an exhaust pipe to suck a large volume of air in the above mentioned space. With the arrangement of this kind, an air flow (see 103 in FIG. 13) in the direction of the exhaust pipe 18 is produced around the gap 9 by sucking through the exhaust pipe 18 a large amount of air from the space between the lid and the screen. As a result of this, the gas leaking out through the space 9 between the ladle 1 and the lid 6 follows the flow 103 of the large amount of air and flows in the direction of the exhaust pipe 18. Thus the gas leaking out from nearby shops can be prevented from damaging the environment. At the same time, as long as the external air is flowing in the direction of the exhaust pipe 18, it is possible to prevent the air from flowing from the gap between the ladle and the lid into the ladle and from lowering the temperature in the ladle or from decreasing smelting ability.